Oxygen is needed for the biological decomposition of organic pollutants contained in waste waters. In customary basins of the biological activated-sludge sewage treatment plants, the introduction of oxygen into the waste water is effected either by blowing compressed air into a depth of a few meters below water level, or by a surface ventilation. In isolated cases, pure oxygen is used instead of air. The waste water absorbs the oxygen in the dissolved form. In connection with the introduction of air and/or oxygen below the water surface, the blow-in depth and the size of the gas bubbles are of significance for the effectiveness of the plant. A higher pressure combined with a greater blow-in depth enhances the gas charge; likewise, many small bubbles are more advantageous, as compared to a few large bubbles, because of the greater total contact surface of gas/water. In addition, the contact time plays a role, and specifically the gas bubbles having been introduced into a greater depth have available--as a result of the longer ascending path to the water surface--a correspondingly longer contact time. On the other hand, it must be taken into account, of course, that the size of the gas bubbles, and therewith the contact time, is dependent upon the respective water depth and the pressure prevailing there.
The energy consumption needed for the gas charge in the known waste water ventilating basins is on the order of 0.7-1.5 kWh per kilogram of oxygen being introduced and constitutes a significant portion of the operational costs of a sewage treatment plant.
In conceptualizing a sewage ventilating basin, the following physical conditions must be taken into consideration for an optimal oxygen introduction or charge:
--the presence in minute bubbles of the gas to be added
--a high pressure during the contact time
--a long contact time
--the uniform reach or inclusion of the entire quantity of sewage.
A good, minute-bubble gas introduction method consists in pressing the gas being supplied by a blower or compressor through a finely porous body, which may have any desired shape, under water into the basin. The porous body may consist, for example, of a ceramic, or a corrosion-resistant material. The uplift or upward pressure velocity of the gas bubbles issuing from the porous surface into the water depends upon the size of the bubbles and the viscosity of the sewage. Thus, for example, smaller gas bubbles are frequently passed by larger ones so that, during their rising in stagnant water, larger and fewer bubbles appear at the surface than are produced below, which is disadvantageous because of the upwardly decreasing total contact surface of gas/water.
This natural phenomenon may be countered, for example, in that the large bubbles are dispersed over and over again by means of producing a high turbulence in the water.
A turbulence may be brought about by deflecting the path of the bubbles with static means, for example by installing tilted baffle plates at whose edges turbulent zones are generated. More effective, however, would be mechanical devices, such as, for example, oppositely-directed paddles or propellers which, in addition to generating turbulence, will also directly break up large bubbles.
The pressure at which the oxygen introduction or charge is to take place is determined, in an open basin, by the water depth wherein, as is well known, the pressure increases linearly with the depth. The pressure in the gas bubbles corresponds precisely to the pressure of the ambient water in the respective depth. In an ascending gas bubble, therefore, the pressure decreases and the volume increases at the same time; in other words, rising bubbles expand and the contact surface of the individual bubble also increases.
Basically, two different possibilities exist for obtaining a high pressure in sewage, namely on the one hand the generation of high pressure within closed containers by means of either hydraulic or pneumatic systems, and on the other hand the utilization of great water depths in ventilating towers or ventilating shafts.
While in sewage towers the raw sewage as well as the residual sludge must be pumped up to the pressure head of the tower, this expenditure may be obviated in cases where a subterranean shaft can be sunk. Here the sewage inflow, the residual sludge feed, and the water drainage will hardly occasion any energy costs other than those which arise in the customary flat basin plants. Yet the energy expenditure involved in the introduction of oxygen into the water is relatively great.
In a sewage tower and in a deep sewage shaft, the water circulation plays an important part. The cross-sectional dimensions are relatively small as compared to the water depth, and the water does not circulate independently since no high temperatures or differences in density occur. In actual operation it is therefore not sufficient simply to replace any oxygen-enriched water discharging from the top with inflowing water being introduced from above because such inflowing water could practically not flow into the lower zones where the rational oxygen charge or introduction can take place.
For this reason the water circulation must be artificially produced and/or enhanced. When the oxygen is introduced at the tower or shaft bottom, the untreated sewage must be brought to this bottom area, and precisely together with the residual sludge from the subsequent purification. The shaft content must be adjusted or coordinated to the quantity of sewage to provide a residence time of the sewage within the container which is appropriate with respect to the container, corresponding to the decomposition output.
In sewage shafts, the sewage feed line and the air or oxygen line are expediently installed in the interior of the shaft. Instead of causing the water to flow just once upwardly from below and then have it flow into the subsequent purification basin, it may also be advisable to circulate the water being present in the shaft repeatedly in the vertical direction. This may be accomplished, for example, in that the tower or shaft is subdivided into two or more like or unlike shafts by means of vertical walls or pipes, and the circulation is effected with feed pumps. For the purpose of a good intermixture, the raw sewage is advantageously added to the revolution in a downwardly-directed stream. In order to achieve as long as possible a contact time of the gas bubbles in the water, the air is brought to a high pressure by means of a compressor and thereafter introduced into the sewage as much as possible in proximity to the bottom of the shaft and as much as possible in minute bubbles.
This requires a high energy expenditure for the compressor so that the operational costs of known sewage treatment plants with sewage ventilating basins are relatively high.